The invention pertains generally to the field of doppler radars, and, more particularly, to the field of police traffic surveillance radars using doppler shifted radar returns to calculate speed.
Police radars have been in use for many years. Police radars output beams of microwave energy from directional antennas and collect microwave energy reflected from stationary and moving objects. The reflected microwave energy has had its frequency changed by the relative motion between the transmitting antenna and the object from which the energy was reflected by the Doppler shift phenomenon. To determine the relative speeds between the transmitting antenna (which is moving at the same speed as the patrol car or policeman which is supporting the transmitting antenna) and the objects from which the microwave energy is reflected, a sample of the transmitted energy is mixed with the reflected microwave energy in a nonlinear mixing diode. The mix products include upper and lower sidebands which represent sum and difference, respectively, between the transmit frequency and the frequency of the reflected microwave energy. Each target which reflects energy, such as the ground and a moving car, will generate a return signal with a frequency which is established by the speed of the target. The frequencies of these returns are analyzed to determine the speed of the patrol car (from the ground return or other returns from stationary objects) and the speed of the moving target with the strongest return.
The evolution of the designs of police radars to the current state of the art is probably best represented by the Stalker.TM. police traffic radar which is commercially available from the assignee of the present invention, Applied Concepts, Inc. of Richardson, Texas. The Stalker was the first police radar unit to convert the mix products to digital samples and send the digital data to a processor unit for analysis.
The prior art has several areas in which improvements can be made. The police cruiser is a noisy environment with radio frequency interference from the patrol car ignition system and police radio unit prevalent as well as random interference from CB radios in other vehicles. Further, the distance from the processing unit to the antenna unit is often quite large. These large distances cause the cable to pick up more noise, and losses in the cable can degrade the operation of the unit. Therefore, small cable size, good noise immunity and the ability to transmit over large distances the signals from which the patrol speed and target speed is the be calculated are important properties for an improved police radar to have.
One of the biggest problems in police radars is in insuring accurate measurement of patrol car speed and target speed. The law requires that the only target speed that is admissible evidence is the target speed calculated from the strongest signal which is not the return from a stationary object. There are several sources of strong signals that can cause erroneous radios. One is CB radios. Many CB users use illegal linear amplifiers that boost their signal power beyond the legal limit of 5 watts. Further, CB radios that are close to the patrol car can also cause strong radio frequency interference. A way to detect this type of radio frequency interference and eliminate it is desirable in police radars. Another big problem is harmonics of the transmitted signal. The transmitted signal has even order harmonics and odd order harmonics. By far, the strongest harmonic is the 2d order harmonic, and when this harmonic is in the microwave energy that bounces off stationary or moving objects, its signal strength can be quite high and can be mistaken for a target signal. It is highly desirable to have a mechanism to eliminate the even order harmonics during processing of the returned microwave energy.
Another source of errors is intermodulation products generated in the receiving and amplifying circuits. It is highly desirable to have a way of eliminating these intermodulation products. Further, it is highly desirable to have a way of eliminating weak signals which are not valid candidates for patrol speed during processing of the return signals.
Most police radars these days do not provide any indication to the patrolman of the speed of the fastest target in the returned microwave energy if that target is not the strongest target in the radar return since the patrolman is not legally authorized to cite the driver of that target since it is not the strongest target return. It is useful to be able to satisfy the requirement of displaying the speed of the strongest target return while simultaneously displaying the speed of the fastest target in the return. This provides the patrolman with advance warning that a faster target is approaching so that when the strongest target return recedes, he or she can re-aim the radar gun at the fastest target and make it the strongest target return for purposes of citing the driver thereof.
Another problem in prior art police radars is accidental calculation of the wrong patrol car speed. This can happen when, for example, the processor locks onto a harmonic return or when the patrol car comes to a stop and another car stops beside the patrol car and then pulls away. In the latter situation, the relative speed between the patrol car and the car pulling away may be inadvertently selected by the processor as the patrol car speed. It is useful to be able to manually reject an obviously incorrect patrol car speed and force the system to lock onto a different return for calculation of the patrol car speed. Further, if it is not possible to find another candidate for patrol speed, it is useful to have a system which can override the manual input rejecting the currently displayed patrol speed and continue to display that patrol speed.